Mobile communication appliances are increasingly making use of radio-frequency transmission architectures which apply the amplitude variation that is required for modulation of the payload data to the radio-frequency output signal at the output stage of the transmission path. The expression amplitude variation is used in this context not only for high-speed amplitude modulation with respect to the modulation type used for the payload data but also for the relatively slow variation of an output level setting.
Frequently, phase modulation is first of all carried out in transmission paths as a function of the modulation type used and of the data to be transmitted on the transmission signal, with the amplitude of a phase-modulated transmission signal then being varied. The amplitude modulation for the transmission of the payload data is in this case at a considerably higher frequency than the comparatively slow adjustment of the output level.
Transmitters which operate on this principle are referred to, inter alia, as EER transmitters (short for “Envelope Elimination and Restoration”), polar-loop transmitters or polar transmitters. One amplitude modulation option that is frequently used in this case is power adjustment of an amplifier stage by the application of a supply voltage to the amplifier stage. This is frequently done using a so-called series regulator, which essentially varies the supply voltage to the amplifier stage via a controlled transistor with the aid of the amplitude modulation signal.
However, this principle has the disadvantage that a relatively large amount of power is consumed uselessly via the series regulator. Even when the amplifier stage is being operated at the maximum output power, that is to say with the minimum voltage drop across the series regulator, a not inconsiderable amount of heat is produced, depending on the amplitude modulation. Since the amplitude modulation and, in particular, its signal statistics are in fact governed by the mobile communication standard being used, the power losses represent an upper limit on the efficiency of the transmitter.
In order to keep the power losses resulting from the voltage drop in the supply voltage across the voltage regulator as small as possible the input of the voltage regulator is not connected directly to the supply voltage Vbat as described in one embodiment that is known to the inventor, but a voltage regulator 19c is connected between the supply voltage and the input of the voltage regulator 15a. The regulator 19c is used to convert the supply voltage Vbat to a different value, and is also referred to in the following text as a DC/DC converter. In consequence, the series regulator can also be readjusted, thus reducing the voltage drop across the series regulator 15a. If the radio-frequency output power via the radio-frequency amplifier 11 is decreased, as is achieved by reducing the voltage V2 at the supply input to the radio-frequency amplifier, the efficiency of the overall arrangement thus does not decrease so severely. For this purpose, the power level information which is derived from the maximum desired power emitted at the output of the output amplifier 11 is applied to the first control input 131 a of the voltage converter 19c. The series regulator arrangement 15a is also supplied with the radio-frequency amplitude modulation signal at its control input 132a. 
In general, the series regulator 15a has to have a considerably wider signal bandwidth for application of the modulation to the supply voltage to the radio-frequency amplifier 11 than is required just for pure power control of the output power of the radio-frequency amplifier 11. It is expedient to choose the bandwidth for the amplitude modulation signal, that is to say in particular the bandwidth of the series regulator 15a, to be greater by a factor of about 10 than the channel bandwidth of the amplitude modulation being used, in order to reduce distortion.
A further known circuit for supplying voltage to the radio-frequency amplifier 11 is shown in FIG. 9. In the case of this amplifier, the power level information and the amplitude information are contained in the control signal at the input 13 in a DC/DC converter 19b. The so-called amplitude modulation word AW is supplied to the control input, in response to which the converter adjusts its output voltage V2 appropriately. Particularly in the case of clocked DC/DC converters, there are accordingly two mutually contradictory requirements. High efficiency should be achieved on the one hand, with a wide control bandwidth on the other hand. These two requirements are virtually impossible to combine in all relevant mobile radio systems. The signal bandwidth for the amplitude modulation should, as already mentioned, be ten times the modulation bandwidth of the switching elements which are processing the amplitude modulation signal. For a clocked DC/DC converter, this would result in the clock frequency having to be one hundred times as great as the modulation bandwidth. A converter such as this would have to produce spectral components in the modulation sidebands of the radio-frequency spectrum in its output signal, thus necessitating analog post-filtering. This filtering would have to have a high level of stop-band attenuation, which could be achieved only with major difficulty within the DC/DC converter or the voltage converter, owing to the high switching currents that occur.